This disclosure relates to headphones that can also be used as out-loud listening devices where they project sound into the surrounding environment.
Headphones are designed to efficiently deliver relatively low volumes of sound to the ears. If headphones could also be used to play music out loud, they would serve a dual purpose. However headphone drivers are usually highly compliant and headphones do not have much back cavity volume. Headphones thus are not able to produce low frequency sounds at sufficient volume to be useful as out-loud listening devices.